1. Field of the Invention
The present invention relates to the field of catalysis, catalysts, metal catalysts and reactions performed with metal catalysts. The field of the invention also includes to the field of novel catalyst support systems and the use of metal eutectics in a liquid state to support metal catalysts and support chemical reactions therein.
2. Background of the Art
Catalysis is the change in rate of a chemical reaction due to the participation of a substance called a catalyst. Unlike other reagents that participate in the chemical reaction, a catalyst is not consumed by the reaction itself. A catalyst may participate in multiple chemical transformations. Catalysts that speed the reaction are called positive catalysts. Substances that slow a catalyst's effect in a chemical reaction are called inhibitors. Substances that increase the activity of catalysts are called promoters, and substances that deactivate catalysts are called catalytic poisons.
Catalytic reactions have a lower rate-limiting free energy of activation than the corresponding uncatalyzed reaction, resulting in higher reaction rate at the same temperature. However, the mechanistic explanation of catalysis is complex. Catalysts may affect the reaction environment favorably, or bind to the reagents to polarize bonds.
Kinetically, catalytic reactions are typical chemical reactions; i.e., the reaction rate depends on the frequency of contact of the reactants in the rate-determining step. Usually, the catalyst participates in this slowest step, and rates are limited by amount of catalyst and its “activity.” In heterogeneous catalysis, the diffusion of reagents to the surface and diffusion of products from the surface can be rate determining. Analogous events associated with substrate binding and product dissociation apply to homogeneous catalysts.
Although catalysts tend to not be consumed by the reaction itself, they may be inhibited, deactivated, or destroyed by secondary processes. In heterogeneous catalysis, typical secondary processes include coking where the catalyst becomes covered by polymeric side products, products or by-products of the reaction. Additionally, heterogeneous catalysts can dissolve into the solution in a solid-liquid system or evaporate in a solid-gas system.
The chemical nature of catalysts is as diverse as catalysis itself, although some generalizations can be made. Proton acids are probably the most widely used catalysts, especially for the many reactions involving water, including hydrolysis and its reverse. Multifunctional solids often are catalytically active, e.g., zeolites, alumina, higher-order oxides, graphitic carbon, nanoparticles, nanodots, and facets of bulk materials. Transition metals are often used to catalyze redox reactions (oxidation, hydrogenation). Examples are nickel, such as Raney nickel for hydrogenation, and vanadium(V) oxide for oxidation of sulfur dioxide into sulfur trioxide. Many catalytic processes, especially those used in organic synthesis, require so called “late transition metals”, which include palladium, platinum, gold, ruthenium, rhodium, and iridium.
Heterogeneous catalysts are typically “supported,” which means that the catalyst is dispersed on a second material that enhances the effectiveness or minimizes their cost. Sometimes the support is merely a surface on which the catalyst is spread to increase the surface area. More often, the support and the catalyst interact, affecting the catalytic reaction. Supports are often porous materials with a high surface area, most commonly alumina or various kinds of activated carbon. Specialized supports include silicon dioxide, titanium dioxide, calcium carbonate, and barium sulfate.
Homogeneous catalysts function in the same phase as the reactants, but the mechanistic principles invoked in heterogeneous catalysis are generally applicable. Typically homogeneous catalysts are dissolved in a solvent with the substrates. One example of homogeneous catalysis involves the influence of H+ on the esterification of esters, e.g., methyl acetate from acetic acid and methanol. For inorganic chemists, homogeneous catalysis is often synonymous with organometallic catalysts
Hydrogen is the most abundant element in the universe, but it can be difficult and costly to obtain as a pure gas. The present technologies used for providing hydrogen gas include electrolysis of water to produce hydrogen and oxygen and splitting hydrogen gas from water or alcohols. The ability to pull apart H2O and particularly CH2OH or CH3CH2OH molecules into their constituent atoms is important to creating a hydrogen-based energy economy. To do so in a cheap and energy efficient manner could potentially turn Earth's vast supply of water into a carrier or supply of cheap, clean power.
But most hydrogen gas on earth comes packaged as water, which can be split into oxygen and hydrogen through a process called electrolysis. Electrolysis requires a good deal of electricity, but if renewable fuels generate that power, the process can be carbon neutral. Electrolysis usually requires a catalyst to efficiently split water into oxygen and hydrogen gas, the most common of which is platinum, which is expensive.
Metal catalysts such as Fe, Ni, Co, Me, Pt and complexes thereof are useful in the hydrogen gas forming processes. By metal catalysts, it is not meant that only a single metal, or a metal(s) in a zero valence metal state is used. Compounds, complexes and combinations of metals and different mixtures of crystalline forms or states of metal can be used. For example, the Berkeley Lab team devised a high-valence metal molybdenum-oxo (PY5Me2) for a catalyst for electrolysis.
Many other forms of catalysts are available for use in the many various chemical reactions, including those useful in generating hydrogen.
Low temperature catalytic ethanol conversion over ceria-supported platinum, rhodium, and tin-based nanoparticle systems are described in Mahmoud, Eugene Leo Draine (2010) Low temperature catalytic ethanol conversion over ceria-supported platinum, rhodium, and tin-based nanoparticle systems. Engineer's thesis, California Institute of Technology. http://resolver.caltech.edu/CaltechTHESIS:06102010-171305208. It is there discussed that due to the feasibility of ethanol production in the United States, ethanol has become more attractive as a fuel source and a possible energy carrier within the hydrogen economy. Ethanol can be stored easily in liquid form, and can be internally pre-formed prior to usage in low temperature (200° C.-400° C.) solid acid and polymer electrolyte membrane fuel cells. However, complete electrochemical oxidation of ethanol remains a challenge. Prior research of ethanol reforming at high temperatures (>400° C.) has identified several metallic and oxide-based catalyst systems that improve ethanol conversion, hydrogen production, and catalyst stability. In this study, ceria-supported platinum, rhodium, and tin-based nanoparticle catalyst systems were developed and analyzed in their performance as low-temperature ethanol reforming catalysts for fuel cell applications. Metallic nanoparticle alloys were synthesized with ceria supports to produce the catalyst systems studied. Gas phase byproducts of catalytic ethanol reforming were analyzed for temperature-dependent trends and chemical reaction kinetic parameters. Results of catalytic data indicate that catalyst composition plays a significant role in low-temperature ethanol conversion. Analysis of byproduct yields demonstrate how ethanol steam reforming over bimetallic catalyst systems (platinum-tin and rhodium-tin) results in higher hydrogen selectivity than was yielded over single-metal catalysts. Additionally, oxidative steam reforming results reveal a correlation between catalyst composition, byproduct yield, and ethanol conversion. By analyzing the role of temperature and reactant composition on byproduct yields from ethanol reforming, this study also proposes how these parameters may contribute to optimal catalytic ethanol reforming.
Published US Patent Application Document No. 20090012345 (Al Nashef) provides a potentially economically viable process for the destruction of small to large quantities of halogenated hydrocarbons, their homologous/analogues, and similar hazardous chemicals at ambient conditions using superoxide ion in deep eutectic solvents. The superoxide ion is either electrochemically generated by the reduction of oxygen in deep eutectic solvents or chemically by dissolving Group 1 (alkali metals) or Group 2 (alkaline earth metals) superoxides, e.g. potassium superoxide, in deep eutectic solvents.
Published US Patent Application Document No. 20110015446 (Maurer) describes the use of a supported noble metal catalyst obtainable by applying a sparingly soluble noble metal compound to a support from solution or suspension, and subsequently treating thermally, for preparing olefinically unsaturated carbonyl compounds.
Published US Patent Application Document No. 20100210454 (Epshteyn) is generally directed to a nanocomposite catalyst material for electrochemical devices such as fuel cells, comprising metal nanoparticles impregnated on a conductive support that is coated with a transition metal compound. The metal nanoparticles may comprise platinum; the metal phosphate may comprise tantalum oxyphosphate, niobium oxyphosphate, tantalum oxide, niobium oxide, or any combination thereof; and the conductive support may comprise carbon. In addition, the invention provides for a method of making the catalyst material.
Published US Patent Application Document No. 20090326262 (Wan) relates to improvements in metal utilization in supported, metal-containing catalysts. For example, the invention relates to methods for directing and/or controlling metal deposition onto surfaces of porous substrates. The present invention also relates to methods for preparing catalysts in which a first metal is deposited onto a support (e.g., a porous carbon support) to provide one or more regions of a first metal at the surface of the support, and a second metal is deposited at the surface of the one or more regions of the first metal. Generally, the electropositivity of the first metal (e.g., copper or iron) is greater than the electropositivity of the second metal (e.g., a noble metal such as platinum) and the second metal is deposited at the surface of the one or more regions of the first metal by displacement of the first metal. The disclosure further relates to treated substrates, catalyst precursor structures and catalysts prepared by these methods. The invention further relates to use of catalysts prepared as detailed herein in catalytic oxidation reactions, such as oxidation of a substrate selected from the group consisting of N-(phosphonomethyl)iminodiacetic acid or a salt thereof, formaldehyde, and/or formic acid.
A eutectic point, which occurs in a eutectic mixture or eutectic combination of materials is a mixture of substances, especially an alloy, having the lowest freezing point of all possible mixtures of the substances. To be a eutectic mixture, the combination does not have to be at the eutectic point (the lowest temperature at which the eutectic mixture freezes and thaws), as it is the ratio of materials that defines the eutectic. The perfect eutectic mixture is the one mixture of a set of substances able to dissolve in one another as liquids that, of all such mixtures, liquefies at the lowest temperature. If an arbitrarily chosen liquid mixture of such substances is cooled, a temperature will be reached at which one component will begin to separate in its solid form and will continue to do so as the temperature is further decreased. As this component separates, the remaining liquid continuously becomes richer in the other component, until, eventually, the composition of the liquid reaches a value at which both substances begin to separate simultaneously as an intimate mixture of solids. This composition is the eutectic composition and the temperature at which it solidifies is the eutectic temperature; if the original liquid had the eutectic composition, no solid would separate until the eutectic temperature was reached; then both solids would separate in the same ratio as that in the liquid, while the composition of the remaining liquid, that of the deposited solid, and the temperature all remained unchanged throughout the solidification.
A eutectic system is a mixture of chemical compounds or elements that has a single chemical composition that solidifies at a lower temperature than any other composition made up of the same ingredients. This composition is known as the eutectic composition and the temperature is known as the eutectic temperature. On a phase diagram the intersection of the eutectic temperature and the eutectic composition gives the eutectic point. Not all binary alloys have a eutectic point; for example, in the silver-gold system the melt temperature (liquidus) and freeze temperature (solidus) both increase monotonically as the mix changes from pure silver to pure gold.